Process for cleaning coal by dense medium



April 24, 1962 HIROSABURO OSAWA PROCESS FOR CLEANING COAL BY DENSEMEDIUM Filed Aug. 51, 1953 FIG.I

MATERIAL TO BE SEPARATED +2mm I RAW COAL DEDUSTER 2 Sheets-Sheet 1 WATERFINE DUST WATER D NS E E MEDIUM FLOATINGS I SINKINGS DENSE MEDIUM TSEPARATING DENSE MEDIUM TANK WATER 1 WATER (OVERSIZD WASHING DRAINAGEDRAINAGE WASHING ,(OVERSIZEI I SCREEN scREEN scREEN scREEN I h f I(UNDER) (UNDER) (OVERSIZE) an an (ovERsIzEI N I M lst A scREEN DE SE MEDu lsl B SCREEN II COAL DILUTED DENSE MEDIUM h (UNDERSIZE) (UNDERSIZE)IOVERFLOW) CLASSIFIER FOR HEAVY MATERIAL SlNKlNGS (OVERFLOW) (TO REFUSE)CLASSIFIER HEAVY FOR SAND MATERIAL WATER a SAND WATER CLASSIFYINGTHICKENER FOR IovERFLowI SILT (UNDERFLOW) \J I i SUPER FINE CONDITIONERFOR WATER PARTICLES wATER DENSE MEDIUM (To REUSE) IIQK SAND DENSE MEDIUMJ SEPARATOR FOR FINE PARTICLES WATER} FINE PARTICLES (TO REFUSE) WATERHIROSABURO (To REFUSE I INVENTOR OSAWA ATTORNEYS.

April 24, 1962 HIROSABURO osAwA 3,031,074

PROCESS FOR CLEANING COAL BY DENSE MEDIUM Filed Aug. 51. 1953 2Sheets-Sheet 2 FIG. 2 Is? A IsI B SCREEN SCREEN DILUTED DENSE(UNDERFLOW) SANDIZYEQ) HEAVY MATERIAL 2nd SCREEN WATER (UNDERSIZE)(QVERFLOW) 2nd CLASSIFIER FOR HEAVY MATERIAL (UNDERFLOW) CLASSIFIER FORSAND ovER I (UNDERMW, FLow FIG. 3 SAND +WATER WATER CLASSIFYING TST AlsI B 5 THICKER FOR SILT SCREEN SCREEN UNDERFLOW 1/5. (umslw WATERDILUTED DENSE h MEDIUM To SEFARATOR CONDITIONER FOR OVERFLOW FOR FINEDENSE MEDIUM I I CLASSIFIER FOR I PARTICLES v HEAVY MATERIAL M V RCLASSIFIER L w TIUNIERFLWI HEAVY MATERIAL WATER SAND T0 SEPARATING FORSAND TAN K r( UNDERFLOW) SAND WATER OVERFLOW) CLEAN ER FOR HEAVYMATERIAL SAND WATER UNDERFLOWI HEAVY MATERIAL CLASSIFYING *WATER THICKERFOR SILT Fl G. 4 WATER SUPER 7 r FINE PARTICLES +SAND WATER mun-ED DENSE(UNDERFLOW) CONDITIONER FOR MEDIUM T0 SEPARATOR DENSE MEDIUM CLASSIFIERFOR DENSE MEDIUM T HE V TE To SEPARATING TANK (OVERFLOW) (UNDERFLOVI)and SCREEN HEAVY MATERIAL (OVERSIZE) (uunzamzz E,

CLASSIFYING THICKENER FOR SILT WATER (OVERFLOW) P u UNDERFLOVI v E ARETIOE ES INVENTOR.

A E CONDITIONER FOR HIROSABURO OSAWA DENSE MEDIUM To SEPARATER FOR FINEPARTICLES Foams: MEDIUM M To SEPARATING TANK ATTORNEYS 3,031,074 PROCESSFOR CLEANING COAL BY DENSE MEDIUM Hirosaburo Osawa, 698 Z-chomeKugayama, Suginami-ku, Tokyo, Japan Filed Aug. 31, 1953, Ser. No.377,652 Claims priority, application Japan Aug. 30, 1952 4 Claims. (Cl.209-12) The present invention relates to improvements in or relating toa method for cleaning coal by dense medium.

An object of the present invention is to provide a process of preparinga dense medium enabling high separating efficiency.

Another object of the present invention is the provision of a process ofpreparing a dense medium and of recovery of medium material enablinghigh recovery efficiency and adapted to be reused repeatedly.

A further object of the present invention is to provide a process ofpreparing a dense medium and cleaning coal enabling low cleaning and lowinstallation costs.

Still another object is the provision of a process for cleaning coal orsimilar materials of a size finer than the smallest size of coal whichcan be cleaned by the usual heavy liquid separation process of thistype.

Other objects, features and advantages of the present invention will beapparent from the hereinafter detailed illustration in thespecification.

In order to prepare a stable dense medium from fine particles ofchemical compound of iron or other heavy material for cleaning coal bydense medium, it will be necessary for the grain size to be extremelyfine. However, in addition to the high pulverizing cost, the preparationof heavy material of a grain size of such an extreme fineness rendersalso difiicult the separation from the heavy material of the extremefine matter mingled in the dense medium, in the separation treatment forrecovering used dense medium, which results in lowering the recoveryetficiency of the heavy material. The higher the fineness of the saidextremely fine particles becomes, the more this phenomenon is mademanifest. Hence, it has already been suggested to use the magneticseparator to .enhance the recovery efficiency of the heavy material. In

the said process, however, the available heavy material is limited onlyto the magnetizable material. In addition, the use of a magneticseparator, requires a magnetizer,

nited States ate nt .demagnetizer and so forth. Further, the densemedium is unstable and forms a hard precipitation while standing atrest. For avoiding these obstacles, specific devices and apparatusbecome necessary. Thus, such a process is not only unsuitable for thepreparation of a stable dense medium, but there is also the disadvantageof increasing the installation cost for cleaning coal. A process is alsowell known, in which a coarse grain heavy material is used, with whichsome bentonite or other stabiliser is admixed to stabilise the densityof the liquid. However, there is likewise a disadvantage in thisprocess, in that if one intends to obtain a stable dense medium inaccordance with this process, the viscosity of the dense medium becomesso high that even thecleaning of small lumps of coal will be difficult,the separating elficiency becomes low, and the recovery of thestabiliser is not satisfactory.

In an attempt to seek an improved coal cleaning process as a solution tothe aforementioned deficiencies found in the usual processes, theinventor herein has investigated the cleaning of coal by a dense medium.It has been found from the results of such investigation that when adense medium is produced by mixing in adequate proportions of pyritecinder (sp. gr. 3.0-5.0), siderite cinder (sp. gr. 3.0-5.0), otheriron-bearing slag (sp. gr. 3.0-4.0), limonite (sp. gr. 3-3.8), hematite(sp. gr. 3.5-5.2), magnetite (sp. gr. 3.5-5.2), pyrrhotite (sp. gr.

ice

3.2-4.6), galena (sp. gr. 5.0-7.5) or ferrosilicon (sp. gr. 5.0-7.0),having grain size ranging between approximately 300 and 60 micron, silt,finely divided material of shale, coaly shale or sandy shale, havinggrain size ranging between 50 and 5 micron, and water, the dense mediumthus obtained indicates satisfactory stability in a specific gravity of1.35 to 2.0, is never found to precipitate hard precipitates and is oflow viscosity, thus enabling even the separation of fine coals, and thata dense medium of high cleaning efliciency can be obtained therefrom atrelatively low cost, and moreover, when the dense medium is diluted withWater, the homogeneity of liquid density is lost immediately, and theheavy material and silt present distinct strata-forming phenomenon,which enables easy and perfect separation of the said two materials.

The present invention is characterized by the following facts: Therequired dense medium is prepared from a specific heavy material of aspecial grain size, silt and water. Raw coal is subjected to theseparation by specific gravity, by using said medium. Then, the densemedium adhering to thus cleaned or separated product is Washed withWater to eliminate it. At the same time, in the recovery of heavymaterial and silt from diluted dense medium resulting from the Washingwater, the heavy material is recovered on one hand from said diluteddense medium by using a screen and a classifier or similar machine, andthe desired silt is recovered on the other hand by using a screen or aclassifier for sand and classifying thickener for silt. By utilizing theheavy material and silt thus recovered, the required dense medium isprepared and adapted to be reused.

The required grain size of the heavy material in the dense medium variesin accordance with the behavior, type and so forth of the raw coal to becleaned therewith. It has been ascertained, however, from the result ofnumerous experiments that the appropriate grain for the present purposeranges from about 300 micron in maximum diameter to approximately. 60micron in minimum diameter. It is, however, preferable to choose thedifference as small as possible between the maximum and the minimumgrain size of heavy material from the point of the recovery, when saidheavy material is used for cleaning coal, because this serves forseparating inutile fine particles mingled in the heavy material used,perfect recovering said dense material and for decreasing theinstallation cost for the cleaning of coal. The term silt as used hereinmeans a material corresponding to one-of 50 to 5 micron in graindiameter and having specific gravity of approximately 2.1 to 2.7 whichforms from the'natural or artificial pulverization of shale, coaly shaleor sandy shale. The silt .as considered in ordinary applications resultsfrom the disintegration of shale in the raw coal to be cleaned. Thus,the dens medium to be used in the process of the present invention has afeature that both the specific gravity and the particle size of theheavy material contained therein are greater than those of the silt.Furthermore, the required composition of the said dense medium, thoughit varies remarkably depending upon the required specific gravity of thedense medium, the grain size and specific gravity of the heavy material,is found nearly satisfactory as referred to the result of theexperiment, when the silt contained an amount of more than 10% by volumeof the whole medium particles. In addition, the initial silt generallyrequired is taken from any external source when starting to prepare thedense medium. Once the cleaning sets in, however, the external supply ofsilt is not needed as said silt will come out of coal to be cleaned orheavy material.

Now, when the dense medium is put in a container and diluted with waterof several times of the volume, said dense medium loses immediately thehomogeneity of liquid density and forms a settling zone of the heavymaterial in the lower portion and a liquid zone of silt in the upperportion. Then, the liquid zone of silt forms .gradually an upper clearlayer and this upper clear layer increases in size with the passage oftime, Whereas the resulting zone of silt particles underlying said upperclear layer decreases inversely with increase of said layer and thesettling zone of heavy material does not show any substantial change involume with the time elapsed. This phenomenon indicates that heavymaterial may be separated easily from the diluted dense medium, whilethe water added to the silt during washing operation may be separatedeasily from the silt.

iii

Therefore, according to the present invention, the desired cleaning ofcoal by a dense medium can be carried out exactly, the heavy materialcan be recovered simply and perfectly, and moreover the requiredrecovery of silt can be also carried out.

Thus, the required cleaning of coal by the dense medium can be carriedout with low cleaning and installation costs; and with high efliciencyand high recovery of valuable dense medium material.

Further, in the present invention the dense medium is stable and of lowviscosity, and in the recovery of dense medium inutile fine particlesmingled in this liquid medium can also be separated.

In order that the present invention may be more clearly understood,several embodiments thereof will now be described by way of examples andwith reference to the accompanying schematic flow sheets, in which:

FIG. 1 shows schematically a fundamental flow sheet illustrating onemethod of operating cleaning coal by dense medium according to thepresent invention.

FIGS. 2, 3 and 4 show respectively a schematic flow sheet of a modifiedembodiment of the present invention.

In FIG. 1, raw coal passes through the deduster where dust (particlesize below about 1-2 mm.) is removed. An ordinary screen is used as thededuster. The screen is preferably washed with water to render thededusting effective. The dedusted raw coal enters the separating tankcontaining the dense medium, where it is separated into floatings, i.e.,coal, and sinkings. The fioatings and sinkings are removed separatelyfrom the separating tank, each being led to a washing machine whereineach is irrigated with water. Then, each is sent to its respectivedestination, as shown in the flow sheet. The dense medium from thedrainage screen is usually sent directly to a conditioner for thisparticular medium. Furthermore, in order to ensure an efiectiveutilization of the plant said drainage and washing operations may becarried out in one and the same screen, and besides both coal and shalemay be treated in one screen, by dividing the screen surface with apartition wall longitudinally. The resultant diluted dense medium issubjected to an operation for removing inutile fine particles mingledtherein and then to a thickening operation. The total quantity ofdiluted dense medium is not always necessarily to be subjected to thisoperation. The treatment and the quantity to be treated depend on theamount and behavior of the inutile material mingled in this liquid aswell as on the type and capacity of the machine employed in thisprocedure. The procedure may vary according to the machine used and thespecific gravity, and grain size of the heavy material.

In either of the four embodiments shown in this specification, thediluted dense medium is first passed through the first A screen andfirst B screen to remove larger particles than dense medium materialtherefrom. These screens may be provided separately, as illustrated inthe flow sheet, from the above mentioned washing screen. However, forthe sake of eflicient utilization of the equipment, the screen may alsobe provided in the lower stage of the washing machine as a part thereof.

The liquid which passes through these screens is introduced to aclassifier or similar machine for recovering heavy material and also forremoving inutile fine powder contained therein. As such a classifier, arising current hydraulic classifier or a classifying thickener isusually used. However, other machines such as cyclone separator andcentrifugal separator may also be employed.

In the fundamental flow sheet of the embodiment of the present inventionshown in FIG. 1, it is preferred that the ratio between the maximumdiameter of grain (d of the fine particles passed through the first Ascreen and the first B screen and the minimum diameter of grain (d ofthe heavy material, i.e. (ti /d is equivalent or less than the equalfalling ratio in the water (R), i.e.

Ai-t

between heavy material (sp. gr.=A in use and shale (sp. gr.=A in the rawcoal.

In the flow sheets as shown in FIGS. 2 and 3 it is preferred that thesaid ratio (d /d is equivalent or larger than the said equal fallingratio, i.e. (R).

Now, the embodiments as shown in each figure will be described asfollows:

In the case as illustrated in FIG. 1, the underflow from the classifierfor heavy material (consisting of heavy material and water) enters theconditioner for dense medium, and the overflow from said classifier(consisting of all the fine particles plus water which have passedthrough the first A screen and first B screen and silt) enters theclassifier for sand, in which the fine particles larger than therequired silt are removed. The remaining silt then enters the thickenerfor silt. In the thickener fine particles finer than the required siltto be recovered are removed as overflow and the silt is led to theconditioner for dense medium after being thickened to the desiredconcentration. The said heavy material, silt and water led into thisconditioner for dense medium are conditioned therein and the requireddense medium thus obtained is transmitted to the separating tank bymeans of a pump and then used again as a dense medium.

In the case of FIG. 2, each of the underflows of the first and secondclassifiers for heavy material consisting respectively of heavy materialand water enters the conditioner for dense medium, while the overflow ofthe second classifier for heavy material is treated as illustrated inthe figure like the overflow of the classifier for heavy material shownin FIG. 1, and is conditioned to the required dense medium in theconditioner for dense medium and is used again as in the case of FIG. 1.

In the case of FIG. 3, the heavy material having been cleaned in thecleaner for heavy material enters the conditioner for dense medium,while the overflow from the classifier for sand is treated like theoverflow in FIG. 1, and the dense medium is conditioned and then usedagain as in the case of FIG. 1.

FIG. 4 shows the case of using a screen in place of a. classifier forsand, therefore the undersize component passing this screen is fed to aclassifying thickener for silt and the oversize component is fed to aseparator for fine particles as described in FIG. 1.

The dense medium used is conditioned to the required specific gravityand behavior in the conditioner for dense medium and is transmitted tothe separating tank through a pump. The supplementing of heavy materialis usually made in this conditioner.

The following examples illustrate the way in which the process accordingto the present invention may be carried out in practice.

Example 1 59 kg. of powdered pyrite cinder produced in sulfuric acidplant, having specific gravity 4.3 and grain size from to micron, 25.3kg. of silt formed by the disintegration of coal seam shale, havingspecific gravity 2.4 and a grain size from 10 to 50 micron and 75.7 kg.of water were mixed with each other. Thereby, 100 liter of dense mediumhaving specific gravity'of 1.6 was produced. The viscosity of this denseliquid medium was lower than that of aqueous solution of calciumchloride having specific gravity of 1.4, and the liquid density hadrequired stability. A bituminous coal of a grain diameter of about 60-2mm. containing 42.27 percent ash was subjected to cleaning as a sample,the result of cleaning being as follows:

Further, above floatings and sinkings were subjected to the sink andfloat test, using aqueous solution of zinc chloride having specificgravity of 1.6, the result being as follows:

Then, the dense medium adhered to the separated product was washed withwater of a quantity five times that of the dense medium adhered to theseparated product; the diluted dense medium thus obtained was screenedthrough a screen having meshes of approximately 150 micron; theundersize was introduced to the hydraulic classifier, i.e. a recoveringmachine for heavy material, the upward current velocity was adjusted to70 cm. per minute. By the inspection of the overflow, the presence ofheavy material was not observed. In order to recover silt from theoverflow, it was screened in a screen of about 50 micron and theundersize was introduced to the classifying thickener, where an upwardcurrent of about 0.3 cm. per minute was given, and its overflow wasrejected. Then, the underflow from the said classifier for recoveringheavy material and the underflow from the thickener for recovering siltwere suitably mixed and conditioned, so that the composition consistingof heavy material, silt and water was made equal to that of said useddense medium. Thus, a dense medium entirely equal to the original densemedium in the specific gravity, viscosity, the stability of liquiddensity and the like was obtained. The whole operations from thecleaning to recovery were carried out repeatedly by using the densemedium thus obtained. The behavior of this dense medium showed the sameresult as in the above mentioned case.

Example 2 70.8 kg. of galena having specific gravity of 6.4 and grainsize of 61-74 micron, 35.9 kg. of silt having specific gravity of 2.3and grain size of 5 to 50 micron and 73.7 kg. of water were mixed, and100 liter of dense medium of specific gravity of 1.8 was obtained. Theviscosity of the dense medium was lower than that of aqueous zincchloride solution having specific gravity of 1.8 and its liquid densitypossessed the stability as required. The cleaning coal by dense mediumwas carried out on a high bituminous coal specimen having a size ofabout 2-60 mm. and ash content of 38.05 percent, the result thereofbeing as follows:

The floatings (clean coal) and sinkings were subjected to the sinkingtest, using aqueous solution of zinc chloride with sp. gr. 1.8, theresult being as follows:

Specific Specific gravity less gravity Grade than 1.8, higher thanpercent 1.8, percent Floatings (clean coal) 99. 5 0.5 sinkings 0.2 99. 8

Then, the dense medium adhered to the cleaned product Was washed withwater to remove it, and as in the case of Example 1, the underflow fromthe classifier for recovering dense medium and that from the thickenerfor recovering silt were mixed properly for conditioning thecomposition. Thereby the required dense medium was obtained. With thisdense medium, another cleaning coal operation was carried outrepeatedly, the result of which was entirely the same as that had beenobtained in the cleaning with above mentioned dense medium.

The above examples are illustrated with respect to two modes ofembodiments of the present invention by way of examples. Further, withsiderite cinder, iron-bearing slag, limonite, hematite, magnetite,pyrrhotite, ferosilicon other than the above mentioned pyrite cinder andgalena, similar result may be obtained.

The dense medium above described is composed of approximately: to 65percent of the heavy material, 3 to 32 percent of silt, and 17 to 3percent of water by volume.

What I claim is:

1. A process for cleaning coal comprising preparing a dense cleaningmedium, said medium comprising finely divided inorganic material havinga particle size of from 60 to 300 microns, and a specific gravity offrom 3.0 to 7.5; finely divided inorganic material having a particlesize of from 5 to 50 microns and a specific gravity of from 2.1 to 2.7and water in proportions of approximately 8065 percent of the firstmentioned material, 3 to 32 percent of the second mentioned material and17 to 3 percent of water by volume to provide said medium with aspecific gravity of from 1.35 to 2.0; mixing coal with said cleaningmedium, to provide sinkings and floatings each admixed with medium;separating said sinkings and floatings; washing and screening thefioatings to recover the floatings; screening the diluted medium torecover additional fioatings of a size larger than the particle size ofsaid medium; diluting the medium to break the homogeneity thereof;classifying the medium received from said screenings to separate theheavy and light portions of said medium and reconstituting saidseparated heavy and light portions of said medium with water to providesaid dense cleaning medium.

2. A process as in claim 1, wherein the separated sinkings admixed withmedium is washed and screened to recover diluted medium; and combiningthe said diluted medium with the diluted medium recovered from thewashing and screening of said floatings.

3. A process as in claim 2, wherein the overflow from said classifyingstep is passed to a sand classifier, the overflow of said sandclassifier being passed to a second classifier, separating lines fromthe light portions of said medium in said second classifier and passingthe separated light portion of said medium for reconstitution with theheavy portion of said medium.

4. A process for cleaning coal comprising preparing a dense cleaningmedium having a specific gravity of from 1.35 to 2.0, said mediumcomprising a mixture of heavy particles having a specific gravity offrom 3.0 to 7.5 and a particle size from 60 to 300 microns, lightparticles having a specific gravity of from 2.1 to 2.7 and a particlesize from 5 to 50 microns, and water, in proportions of approximately80-65 percent of heavy particles, 3 to 32 percent of light particles,and 17 to 3 percent of water by volume; mixing said medium with thecoal; separating the :mixture into fioatings associated with medium andsinkings associated with medium; washing and screening the fioatings torecover fioatings; Washing and screening the sinkings to recover themedium associated therewith, combining medium recovered from saidfioatings and sinkings, diluting the combined medium to break thehomogeneity thereof, classifying the combined medium to recover theheavy portion of said medium; screening the overflow of said classifyingstep; classifying the underfiow of said last screening step to recoveras underflow the light portion of said medium, and recombining therecovered heavy and light portions of said medium for recycling.

UNITED STATES PATENTS Rakowsky Feb. 13, 1940 Pearson July 2, 1940 DavisDec. 14, 1943 Marsh Feb. 7, 1950 Driessen Oct. 6, 1953 Vogel Aug. 17,1954 Maust Sept. 28, 1954 FOREIGN PATENTS Great Britain Jan. 20, 1949

1. A PROCESS FOR CLEANING COAL COMPRISING PREPARING A DENSE CLEANINGMEDIUM, SAID MEDIUM COMPRISING FINELY DIVIDED INORGANIC MATERIAL HAVINGA PARTICLE SIZE OF FROM 60 TO 300 MICRONS, AND A SPECIFIC GRAVITY OFFROM 3.0 TO 7.5; FINELY DIVIDED INORGANIC MATERIAL HAVING A PARTICLESIZE OF FROM 5 TO 50 MICRONS AND A SPECIFIC GRAVITY OF FROM 2.1 TO 2.7AND WATER IN PROPORTIONS OF APPROXIMATELY 80-65 PERCENT OF THE FIRSTMENTIONED MATERIAL, 3 TO 32 PERCENT OF THE SECOND MENTIONED MATERIAL AND17 TO 3 PERCENT OF WATER BY VOLUME TO PROVIDE SAID MEDIUM WITH ASPECIFIC GRAVITY OF FROM 1.35 TO 2.0; MIXING COAL WITH SAID CLEANINGMEDIUM, TO PROVIDE SINKINGS AND FLOATINGS EACH AD-